At its default clock speed the 3.2GHz Phenom II X2 555 BE is only 100MHz faster than the X2 550, but being a Black Edition the default clock speed matter all that much.  What matters is the overclock that can be reached and that is exactly where Bjorn3D started their testing.  First off they unlocked the two unused cores and tried to boot into Windows without increasing the voltage. The result was a completely stable chip that closely resembled a X4 955.  They then started upping voltages and clock speeds and hit the limit that their cooler could handle at a hair over 4GHz @ 1.464V on the core.  Check out the benchmark results in the full review.
“AMD is always adding new chips to their lineup to fill any gap that you can imagine. Today we have the Phenom II X2 555. This chip comes clocked slightly higher than its younger brother, the 550. This stock clock will show some improvement, but who runs a Black Edition CPU at its rated speed? The whole reason for buying this chip is its unlocked multiplier for overclocking. The Phenom II also provides the L3 cache that the Athlon II lineup lacks, adding some additional performance. Since the start of the Phenom II manufacturing AMD has been getting better at manufacturing these chips. This is leading to an amazing unlock rate for the disabled cores on these chips. Armed with our 890 GX chipset board we will attempt to unlock the two hidden cores, and get this chip to extreme clocks. Lets dive into what should be a good time!”

Here are some more Processor articles from around the web:

Click Here to go to Processors  Processors